1. Field of the Invention
The present invention relates to a laminated piezoelectric actuator.
2. Description of the Related Art
Conventionally, laminated piezoelectric actuators have been used as actuators for operating movable parts of a variety of instruments such as electronically controlled suspensions. The laminated piezoelectric actuators are displaced by a voltage application. A laminated piezoelectric actuator is integrated in the following manner. A green sheet is formed from a raw material in which a lead zirconate titanate (i.e., PZT) powder is mixed with an organic binder, a plasticizer, an organic solvent, and the like. The PZT powder serves as a piezoelectric material. Then, the surfaces of the green sheet are coated with a conductive paste. The coated green sheets are laminated in a plurality of pieces, and are thereafter calcined at a predetermined temperature. Thus, a laminated piezoelectric actuator is integrated in which piezoelectric plates and inner electrode layers are laminated alternately. When electricity is supplied to the inner electrode layers, the piezoelectric plates are extended in the axial direction to operate as an actuator. Hence, it is possible to expect a highly accurate applied voltage-displacement characteristic.
In actuators in which the displacements are caused by applying strong electric fields, it is important that the piezoelectric characteristics are reliable. In the conventional laminated piezoelectric actuators which are mainly composed of PZT, there is a problem in that, when they are operated by applying electric fields repeatedly after they are subjected to a polarization treatment with an electric field in advance, the piezoelectric characteristics, such as the piezoelectric constants and displacements, have been degraded as the number of electric field applications is increased. It is believed that the disadvantage occurs as follows. The concentration distributions of the constituent elements of the piezoelectric plates which are subjected to the polarization treatment, especially, the localization of the oxygen elements localized on the side of the positive electrode is relieved by applying strong electric fields repeatedly (Journal of the Ceramic Society of Japan 107 [9] pp. 827-831).
The present invention has been developed in view of such a problem. It is therefore an object of the present invention to provide a laminated piezoelectric actuator whose piezoelectric characteristics are degraded less when it is operated by applying an electric field repeatedly, namely, whose highly accurate applied voltage-displacement characteristic can be maintained.
A laminated piezoelectric actuator according to the present invention comprises: piezoelectric plates; and inner electrode layers composed of an electrode material; the piezoelectric plates and the inner electrode layers laminated alternately; and the electrode material including a metallic component and an electrically conductive oxide. Note that the electrically conductive oxide can naturally be not only simple oxides but also composite oxides. When the electrically conductive oxide, in which the oxygen ions are made into carriers, is added to the electrode material, oxygen is supplied into the operating piezoelectric plates from the inner electrode layers. As a result, the piezoelectric plates are inhibited from degrading.
In the laminated piezoelectric actuator according to the present invention, the electrically conductive oxide, in which the oxygen ions work as carriers, is added to the electrode material of the inner electrode layers. Accordingly, even when strong electric fields are applied to the present laminated piezoelectric actuator repeatedly, the piezoelectric characteristics of the piezoelectric plates, which includes a PZT composite oxide, are inhibited from degrading. Consequently, it is possible to maintain the reliability as a laminated piezoelectric actuator. The advantage results from the fact that it is accompanied by the repetitive operation that the electrically conductive oxide, which is added in the inner electrode layers, supplies oxygen to the inside of the piezoelectric plates. As a result, it is possible to suppress the lowering of the oxygen concentration due to the repetitive operations.